The invention relates generally to Bourdon tube instruments such as pressure gauges, and, more particularly, to a Bourdon tube gauge in which the effects of variations in the ambient temperature are substantially compensated.
U.S. Pat. No. 3,641,820, issued Feb. 15, 1972 to Bissell, describes a relatively simple and inexpensive Bourdon tube pressure gauge in which the free end of the Bourdon tube is provided with an integral pointer which is movable relative to a calibrated dial plate for indicating the value of pressure being sensed. This pressure gauge is not temperature compensated, and thus will have a temperature-induced error when the ambient temperature varies from the ambient temperature at which the pressure gauge was calibrated. To correct for these temperature-induced errors, the ambient temperature must be measured and correction charts utilized to obtain the correct pressure.
In temperature-compensated Bourdon tube pressure gauges, such as those described in U.S. Pat. No. 2,387,909, issued Oct. 30, 1945 to Ingham, Jr., and U.S. Pat. No. 3,004,434, issued Oct. 17, 1961 to Heise, the free end of the Bourdon tube is connected to a rotatable pointer by a linkage assembly which includes a bimetallic element for adjusting the linkage in accordance with the ambient temperature so that the pointer indicates on a calibrated dial the correct value of pressure being sensed.
Bourdon tube gauges are also extensively used to monitor the fullness of a fire extinguisher tank, that is, to measure the mass of a fire suppressant fluid within the tank relative to the mass of the fire suppressant fluid when the tank is fully charged. Since the volume of the fire extinguisher tank is fixed, the pressure of the working fluid within the tank will be a measure of the mass of this fluid at any given ambient temperature. However, the fluid pressure indicating a fully charged condition will change when the ambient temperature changes. Thus, when an inexpensive Bourdon tube gauge without temperature compensation is utilized to measure the pressure of the fire suppressant fluid, the ambient temperature must be measured and a correction chart must be utilized not only to correct for temperature-induced errors in the Bourdon tube gauge, but also to convert the actual pressure measured at any particular ambient temperature to the corresponding fluid pressure at a reference ambient temperature.
U.S. Pat. No. 4,143,545, issued Mar. 13, 1979 to Sitabkhan describes a Bourdon tube pressure gauge assembly for measuring the fullness of a fire extinguisher tank. In this assembly, a Bourdon tube is disposed within a sealed container containing a pressurized fluid having substantially the same pressure-temperature response characteristic as the fluid within the fire extinguisher tank which is supplied to the interior of the Bourdon tube. The reference fluid is maintain at the same temperature as the working fluid, so that the differential pressure measured by the Bourdon tube assembly between the interior and exterior surfaces of the Bourdon tube will be an indication of the mass or charge of fluid within the fire extinguisher tank.